In-Sequence Spectrographic Sensor

ABSTRACT

A method of controlling a polishing system includes polishing a substrate at a first polishing station, transporting the substrate to an in-line optical metrology system positioned between the first polishing station and a second polishing station, at the in-line optical metrology system measuring a spectrum reflected from the substrate, and generating a characterizing value from the spectrum, determining that the substrate needs rework based on the characterizing value, returning the substrate to the first polishing station and performing rework of the substrate at the first polishing station; and transporting the substrate to the second polishing station and polishing the substrate at the second polishing station.

TECHNICAL FIELD

This disclosure relates to optical metrology and control of a polishing apparatus.

BACKGROUND

An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive, or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface and planarizing the filler layer. For certain applications, the filler layer is planarized until the top surface of a patterned layer is exposed. A conductive filler layer, for example, can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. After planarization, the portions of the metallic layer remaining between the raised pattern of the insulative layer form vias, plugs, and lines that provide conductive paths between thin film circuits on the substrate. For other applications, such as oxide polishing, the filler layer is planarized until a predetermined thickness is left over the non planar surface. In addition, planarization of the substrate surface is usually required for photolithography.

Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. An abrasive polishing slurry is typically supplied to the surface of the polishing pad.

Variations in the slurry distribution, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations, as well as variations in the initial thickness of the substrate layer, cause variations in the time needed to reach the polishing endpoint. Therefore, determining the polishing endpoint merely as a function of polishing time can lead to overpolishing or underpolishing of the substrate. Various in-situ monitoring techniques, such as optical or eddy current monitoring, can be used to detect a polishing endpoint.

SUMMARY

In some systems, the substrate is monitored in-situ during polishing, e.g., by optically or eddy current techniques. However, existing monitoring techniques may not reliably halt polishing at the desired point. A spectrum from the substrate can be measured by an in-sequence metrology station. That is, the spectrum can be measured while the substrate is still held by the carrier head, but at a metrology station positioned between the polishing stations. A value can be calculated from the spectrum which can be used in controlling a polishing operation at one or more of the polishing stations.

In one aspect, a method of controlling a polishing system includes polishing a substrate at a first polishing station, transporting the substrate to an in-line optical metrology system positioned between the first polishing station and a second polishing station, at the in-line optical metrology system measuring a spectrum reflected from the substrate, and generating a characterizing value from the spectrum, determining that the substrate needs rework based on the characterizing value, returning the substrate to the first polishing station and performing rework of the substrate at the first polishing station; and transporting the substrate to the second polishing station and polishing the substrate at the second polishing station.

Implementations may include one or more of the following features. The substrate may be held by a carrier head and the carrier head is suspended from a track, and transporting the substrate may be performed by moving the carrier head along the track. Generating the characterizing value may include obtaining a plurality of measured spectra with the optical metrology system from a plurality of different measurement spots within an area on the substrate, generating a plurality of values based on the plurality of measured spectra, and combining the values to generate the characterizing value. Generating the plurality of values may include comparing each of the plurality of measured spectra to a reference spectrum to generate a similarity value. The substrate may have a plurality of dies, and the area may be substantially equal to an area of one of the dies. Generating the characterizing value may include at least one of identifying a matching reference spectrum from a library of reference spectra and determining the characterizing value associated with the matching reference spectrum, determining a wavelength or width of a peak or valley in the spectrum, or fitting an optical model to the spectrum. Another spectrum reflected from the substrate may be measured at the in-line optical metrology system positioned between the first polishing station and a second polishing station after performing rework and before transporting the substrate to the second polishing station. Polishing the substrate at the first polishing station may include a filler layer clearing recipe, and polishing the substrate at the second polishing station may include an underlying layer polishing recipe. Polishing the substrate at the first polishing station may be a bulk polishing step of a copper damascene process. Polishing the substrate at the second polishing station may be a dielectric exposure step of a copper damascene process. The substrate may be transported to a cassette before returning the substrate to the first polishing station and performing rework of the substrate at the first polishing station. At least one other substrate may be polished at the first polishing station before returning the substrate to the first polishing station and performing rework of the substrate at the first polishing station.

In another aspect, a polishing system includes a first polishing station including a first support for a first polishing pad, a second polishing station including a second support for a second polishing pad, a carrier head to hold a substrate, the carrier head supported by a support structure and movable between the first polishing station and the second polishing station, an in-line optical metrology system positioned between the first polishing station and the second polishing station, the optical metrology system configured to measure a spectrum reflected from the substrate and generate a characterizing value from the spectrum, and a controller configured to cause the carrier head to move to the first polishing station, to cause the substrate to be polished at the first polishing station, to cause the carrier head to move to the in-line metrology system, to determining whether the substrate needs rework based on the characterizing value, to cause the carrier head to return to the first polishing station and performing rework of the substrate at the first polishing station, to cause the carrier head to move to the second polishing station, and to cause the substrate to be polished at the second polishing station.

Implementations may include one or more of the following features. The support structure may be a track and the carrier head may be movable along the track. The carrier head may be suspended from a carriage on the track. The controller may be configured to cause the carrier head to move to the in-line optical metrology system and to receive another spectrum reflected from the substrate at the in-line optical metrology system after performing rework and before transporting the substrate to the second polishing station. The system may include a cassette and a robot to transfer the substrate from the carrier head to the cassette. The controller may be configured to cause the robot to transport the substrate from the carrier head to the cassette before the substrate is returned to the first polishing station and rework of the substrate is performed at the first polishing station. The controller may be configured to cause polishing of at least one other substrate at the first polishing station before the substrate is returned to the first polishing station and rework of the substrate is performed at the first polishing station.

Implementations can include one or more of the following potential advantages. Polishing endpoints can be determined more reliably, and within-wafer non-uniformity (WIWNU) and wafer-to-wafer non-uniformity (WTWNU) can be reduced.

The details of one or more implementations are set forth in the accompanying drawings and the description below. Other aspects, features and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic plan view of an example of a polishing apparatus.

FIG. 2 is a schematic cross-sectional view of an example of a polishing apparatus.

FIG. 3 is a schematic cross-sectional view of an example of an in-sequence optical metrology system.

FIG. 4 illustrates an example spectrum.

FIG. 5 is a schematic cross-sectional view of a wet-process optical metrology system.

FIG. 6 is a schematic cross-sectional view of another implementation of a wet-process optical metrology system.

FIG. 7 is a schematic top view of a substrate.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

As integrated circuits continue to develop, line widths continue to shrink and layers in the integrated circuit continue to accumulate, requiring ever more stringent thickness control. Thus, polishing process control techniques, whether utilizing in-situ monitoring or run-to run process control, face challenges to maintain keep the post-polishing thickness within specification.

For example, when performing in-situ spectrographic monitoring of a multi-layer product substrate, an incident optical beam from the spectrographic monitoring system can penetrate a several dielectric layers before being reflected by metal lines. The reflected beam can thus be a result of the thickness and critical dimensions of multiple layers. A spectrum resulting from such a complex layer stack often presents a significant challenge in determining the thickness of the outermost layer that is being polishing. In addition, the outermost layer thickness is an indirect parameter for process control. This is because in many applications the metal line thickness—a parameter that may be more critical to yield—can vary even if the outermost layer thickness is on target, if other dimensions such as etch depth or critical dimension vary.

A control scheme for determining a polishing endpoint incorporates wet metrology between CMP steps and feedforward or feedback control. The dimensional variations in the substrate are captured after each polishing step at an in-sequence metrology station and used either to determine whether there is a need to rework the substrate, or fed forward or fed back to control the polishing operation or endpoint at a previous or subsequent polishing station.

The polishing apparatus is configured such that a carrier head holds a substrate during polishing at the first and second polishing stations and moves the substrate from the first polishing station to the second polishing station. The in-sequence metrology station is situated to measure the substrate when the carrier head is holding the substrate and when the substrate is not in contact with a polishing pad of either the first polishing station or the second polishing station.

FIG. 1 is a plan view of a chemical mechanical polishing apparatus 100 for processing one or more substrates. The polishing apparatus 100 includes a polishing platform 106 that at least partially supports and houses one or more polishing stations 124. The polishing apparatus 100 also includes a multiplicity of carrier heads 126, each of which is configured to carry a substrate. Each polishing station 124 is adapted to polish a substrate that is retained in a carrier head 126.

The polishing apparatus 100 also includes a loading and unloading station. The loading and unloading station can include one or more load cups 122 adapted to facilitate transfer of a substrate between the carrier heads 126 and a factory interface (not shown) or other device (not shown) by a transfer robot 110. The load cups 122 generally facilitate transfer between the robot 110 and each of the carrier heads 126.

Each polishing station 124 includes a polishing pad 130 supported on a platen 120 (see FIG. 2). The polishing pad 110 can be a two-layer polishing pad with an outer polishing layer 130 a and a softer backing layer 130 b (see FIG. 2).

In some implementations, at least one of the polishing stations 124 is sized such that a plurality of carrier heads 126 can be positioned simultaneously over the polishing pad 130 so that polishing of a plurality of substrates can occur at the same time in the polishing station 124. Thus, a plurality of substrates, e.g., one per carrier head, can be polished simultaneously with the same polishing pad. Alternatively, in some implementations there is just one carrier head 126 per polishing pad 130. In addition, although six carrier heads 126 are shown, more or fewer carrier heads can be depending on the needs of the polishing process and so that the surface area of polishing pad 130 may be used efficiently.

In some implementations, the carrier heads 126 are coupled to a carriage 108 that is mounted to an overhead track 128. The overhead track 128 allows each carriage 108 to be selectively positioned over the polishing stations 124 and the load cups 122. In the implementation depicted in FIG. 1, the overhead track 128 has a circular configuration (shown in phantom) which allows the carriages 108 retaining the carrier heads 126 to be selectively orbited over and/or clear of the load cups 122 and the polishing stations 124. The overhead track 128 may have other configurations including elliptical, oval, linear or other suitable orientation. Alternatively, in some implementations the carrier heads 126 are suspended from a carousel, and rotation of the carousel moves all of the carrier heads simultaneously along a circular path.

Each polishing station 124 of the polishing apparatus 100 can include a port, e.g., at the end of an arm 134, to dispense polishing liquid 136 (see FIG. 2), such as abrasive slurry, onto the polishing pad 130. Each polishing station 124 of the polishing apparatus 100 can also include pad conditioning apparatus 132 to abrade the polishing pad 130 to maintain the polishing pad 130 in a consistent abrasive state.

As shown in FIG. 2, the platen 120 at each polishing station 124 is operable to rotate about an axis 121. For example, a motor 150 can turn a drive shaft 152 to rotate the platen 120.

Each carrier head 126 is operable to hold a substrate 10 against the polishing pad 130. Each carrier head 126 can have independent control of the polishing parameters, for example pressure, associated with each respective substrate. In particular, each carrier head 126 can include a retaining ring 142 to retain the substrate 10 below a flexible membrane 144. Each carrier head 126 also includes a plurality of independently controllable pressurizable chambers defined by the membrane, e.g., three chambers 146 a-146 c, which can apply independently controllable pressurizes to associated zones on the flexible membrane 144 and thus on the substrate 10. Although only three chambers are illustrated in FIG. 2 for ease of illustration, there could be one or two chambers, or four or more chambers, e.g., five chambers.

Each carrier head 126 is suspended from the track 128, and is connected by a drive shaft 154 to a carrier head rotation motor 156 so that the carrier head can rotate about an axis 127. Optionally each carrier head 140 can oscillate laterally, e.g., by driving the carriage 108 on the track 128, or by rotational oscillation of the carousel itself. In operation, the platen is rotated about its central axis 121, and each carrier head is rotated about its central axis 127 and translated laterally across the top surface of the polishing pad. The lateral sweep is in a direction parallel to the polishing surface 212. The lateral sweep can be a linear or arcuate motion.

A controller 190, such as a programmable computer, is connected to each motor 152, 156 to independently control the rotation rate of the platen 120 and the carrier heads 126. For example, each motor can include an encoder that measures the angular position or rotation rate of the associated drive shaft. Similarly, the controller 190 is connected to an actuator in each carriage 108 to independently control the lateral motion of each carrier head 126. For example, each actuator can include a linear encoder that measures the position of the carriage 108 along the track 128.

The controller 190 can include a central processing unit (CPU) 192, a memory 194, and support circuits 196, e.g., input/output circuitry, power supplies, clock circuits, cache, and the like. The memory is connected to the CPU 192. The memory is a non-transitory computable readable medium, and can be one or more readily available memory such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or other form of digital storage. In addition, although illustrated as a single computer, the controller 190 could be a distributed system, e.g., including multiple independently operating processors and memories.

Referring to FIGS. 1 and 3, the polishing apparatus 100 also includes an in-sequence (also referred to as in-line) optical metrology system 160, e.g., a spectrographic metrology system. An in-sequence metrology system is positioned within the polishing apparatus 100, but does not performs measurements during the polishing operation; rather measurements are collected between polishing operations, e.g., while the substrate is being moved from one polishing station to another.

The in-line optical metrology system 160 includes a probe 180 supported on the platform 106 at a position between two of the polishing stations 124, e.g., between two platens 120. In particular, the probe 180 is located at a position such that a carrier head 126 supported by the track 128 can position the substrate 10 over the probe 170. In implementations in which the polishing apparatus 100 include three polishing stations and carries the substrates sequentially from the first polishing station to the second polishing station to the third polishing station, the probe 180 can be positioned between the second and third polishing stations. However, in some implementations the probe 180 is positioned between the first and second polishing stations.

In some implementations there are multiple in-line optical metrology systems 160, e.g., a first in-line optical metrology system has a first probe 180 positioned between the first and second polishing stations, and a second in-line optical metrology system has a second probe 180 positioned between the first and second polishing stations.

In some implementations, the probe 180 should be positioned after a station at which the filler layer is expected to be cleared. For example, where the controller 190 is configured with a recipe to perform bulk polishing (but not clearance) of the filler layer at a first polishing station, clearance of the filler layer at a second polishing station, and removal or clearing of an underlying layer at a third polishing station, the probe 180 can be positioned between the second and third polishing stations. In particular, for a copper polishing process that has bulk copper polishing at the first polishing station, clearance of copper at the second polishing station, and clearing of a barrier layer and a cap layer at the third polishing station, the probe 180 can be positioned between the second and third polishing stations.

The optical metrology system 160 can include a light source 162, a light detector 164, and circuitry 166 for sending and receiving signals between the controller 190 and the light source 162 and light detector 164.

One or more optical fibers can be used to transmit the light from the light source 162 to the optical access in the polishing pad, and to transmit light reflected from the substrate 10 to the detector 164. For example, a bifurcated optical fiber 170 can be used to transmit the light from the light source 162 to the substrate 10 and back to the detector 164. The bifurcated optical fiber can include a trunk 172 having an end in the probe 180 to measure the substrate 10, and two branches 174 and 176 connected to the light source 162 and detector 164, respectively. In some implementations, rather than a bifurcated fiber, two adjacent optical fibers can be used.

In some implementations, the probe 180 holds an end of the trunk 172 of the bifurcated fiber. In operation, the carrier head 126 positions a substrate 10 over the probe 180. Light from the light source 162 is emitted from the end of the trunk 172, reflected by the substrate 10 back into the trunk 172, and the reflected light is received by the detector 164. In some implementations, one or more other optical elements, e.g., a focusing lens, are positioned over the end of the trunk 172, but these may not be necessary.

The probe 180 can include a mechanism to adjust the vertical height of the end the trunk 172, e.g., the vertical distance between the end of the trunk 172 and the top surface of the platform 106. In some implementations, the probe 180 is supported on an actuator system 182 that is configured to move the probe 180 laterally in a plane parallel to the plane of the track 128. The actuator system 182 can be an XY actuator system that includes two independent linear actuators to move probe 180 independently along two orthogonal axes.

The output of the circuitry 166 can be a digital electronic signal that passes to the controller 190 for the optical metrology system. Similarly, the light source 162 can be turned on or off in response to control commands in digital electronic signals that pass from the controller 190 to the optical metrology system 160. Alternatively, the circuitry 166 could communicate with the controller 190 by a wireless signal.

The light source 162 can be operable to emit white light. In one implementation, the white light emitted includes light having wavelengths of 200-800 nanometers. A suitable light source is a xenon lamp or a xenon mercury lamp.

The light detector 164 can be a spectrometer. A spectrometer is an optical instrument for measuring intensity of light over a portion of the electromagnetic spectrum. A suitable spectrometer is a grating spectrometer. Typical output for a spectrometer is the intensity of the light as a function of wavelength (or frequency). FIG. 4 illustrates an example of a measured spectrum 300.

As noted above, the light source 162 and light detector 164 can be connected to a computing device, e.g., the controller 190, operable to control their operation and receive their signals. The computing device can include a microprocessor situated near the polishing apparatus, e.g., a programmable computer. With respect to control, the computing device can, for example, synchronize activation of the light source with the motion of the carrier head 126.

Optionally, the in-sequence metrology system 160 can be a wet metrology system. In a wet-metrology system, measurement of the surface of the substrate is conducted while a layer of liquid covers the portion of the surface being measured. An advantage of wet metrology is that the liquid can have a similar index of refraction as the optical fiber 170. The liquid can provide a homogeneous medium through which light can travel to and from the surface of the film that is to be or that has been polished. The wet metrology system 169 can be configured such that the liquid is flowing during the measurement. A flowing liquid can flush away polishing residue, e.g., slurry, from the surface of the substrate being measured.

FIG. 5 shows an implementation of a wet in-sequence metrology system 160. In this implementation, the trunk 172 of the optical fiber 170 is situated inside a tube 186. A liquid 188, e.g., de-ionized water, can be pumped from a liquid source 189 into and through the tube 186. During the measurement, the substrate 10 can positioned over the end of the optical fiber 170. The height of the substrate 10 relative to the top of the tube 186 and the flow rate of the liquid 188 is selected such that as the liquid 188 overflows the tube 186, the liquid 188 fills the space between the end of the optical fiber 170 and the substrate 10.

Alternatively, as shown in FIG. 6, the carrier head 126 can be lowered into a reservoir defined by a housing 189. Thus, the substrate 10 and a portion of the carrier head 126 can be submerged in a liquid 188, e.g., de-ionized water, in the reservoir. The end of the optical fiber 170 can be submerged in the liquid 188 below the substrate 10.

In either case, in operation, light travels from the light source 162, travels through the liquid 188 to the surface of the substrate 10, is reflected from the surface of the substrate 10, enters the end of the optical fiber, and returns to the detector 164.

Referring to FIG. 7, a typical substrate 10 includes multiple dies 12. In some implementations, the controller 190 causes the substrate 10 and the probe 180 to undergo relative motion so that the optical metrology system 160 can make multiple measurements within an area 18 on the substrate 10. In particular, the optical metrology system 160 can take multiple measurements at spots 184 (only one spot is shown on FIG. 5 for clarity) that are spread out with a substantially uniform density over the area 18. The area 18 can be equivalent to the area of a die 12. In some implementations, the die 12 (and the area 18) can be considered to include half of any adjacent scribe line. In some implementations, at least one-hundred measurements are made within the area 18. For example, if a die is 1 cm on a side, then the measurements can be made at 1 mm intervals across the area. The edges of the area 18 need not be aligned with the edges of a particular die 12 on the substrate.

In some implementations, the XY actuator system 182 causes the measurement spot 184 of the probe 180 to traverse a path across the area 18 on the substrate 10 while the carrier head 126 holds the substrate 10 in a fixed position (relative to the platform 106). For example, the XY actuator system 182 can cause the measurement spot 184 to traverse a path which traverses the area 18 on a plurality of evenly spaced parallel line segments. This permits the optical metrology system 160 to take measurements that are evenly spaced over the area 18.

In some implementations, there is no actuator system 182, and the probe 180 remains stationary (relative to the platform 106) while the carrier head 126 moves to cause the measurement spot 184 to traverse the area 18. For example, the carrier head could undergo a combination of rotation (from motor 156) translation (from carriage 108 moving along track 128) to cause the measurement spot 184 to traverse the area 18. For example, the carrier head 126 can rotate while carriage 108 causes the center of the substrate to move outwardly from the probe 180, which causes the measurement spot 184 to traverse a spiral path on the substrate 10. By making measurements while the spot 184 is over the area 18, measurements can be made at a substantially uniform density over the area 18.

In some implementations, the relative motion is caused by a combination of motion of the carrier head 126 and motion of the probe 180, e.g., rotation of the carrier head 126 and linear translation of the probe 180.

The controller 190 receives a signal from the optical metrology system 160 that carries information describing a spectrum of the light received by the light detector for each flash of the light source or time frame of the detector. For each measured spectrum, a characterizing value can be calculated from the measured spectrum. The characterizing value can be used in controlling a polishing operation at one or more of the polishing stations.

One technique to calculate a characterizing value is, for each measured spectrum, to identify a matching reference spectrum from a library of reference spectra. Each reference spectrum in the library can have an associated characterizing value, e.g., a thickness value or an index value indicating the time or number of platen rotations at which the reference spectrum is expected to occur. By determining the associated characterizing value for the matching reference spectrum, a characterizing value can be generated. This technique is described in U.S. Patent Publication No. 2010-0217430, which is incorporated by reference. Another technique is to analyze a characteristic of a spectral feature from the measured spectrum, e.g., a wavelength or width of a peak or valley in the measured spectrum. The wavelength or width value of the feature from the measured spectrum provides the characterizing value. This technique is described in U.S. Patent Publication No. 2011-0256805, which is incorporated by reference. Another technique is to fit an optical model to the measured spectrum. In particular, a parameter of the optical model is optimized to provide the best fit of the model to the measured spectrum. The parameter value generated for the measured spectrum generates the characterizing value. This technique is described in U.S. Patent Application No. 61/608,284, filed Mar. 8, 2012, which is incorporated by reference. Another technique is to perform a Fourier transform of the measured spectrum. A position of one of the peaks from the transformed spectrum is measured. The position value generated for measured spectrum generates the characterizing value. This technique is described in U.S. patent application Ser. No. 13/454,002, filed Apr. 23, 2012, which is incorporated by reference.

As noted above, the characterizing value can be used in controlling a polishing operation at one or more of the polishing stations. The controller can, for example, calculate the characterizing value and adjust the polishing time, polishing pressure, or polishing endpoint of: (i) the previous polishing step, i.e., for a subsequent substrate at the polishing station that the substrate being measured just left, (ii) the subsequent polishing step, i.e., at the polishing station to which the substrate being measured will be transferred, or (iii) both of items (i) and (ii), based on the characterizing value.

In some implementations, prior to the first CMP step, substrate dimension information (layer thickness, critical dimensions) from upstream non-polishing steps, if available, is fed forward to the controller 190.

After a CMP step, the substrate is measured using wet metrology at the in-sequence metrology station 180 located between the polishing station at which the substrate was polishing and the next polishing station. A characterizing value, e.g., layer thickness or copper line critical dimension, is captured and sent to the controller.

In some implementations, the controller 190 uses the characterizing value to adjust the polishing operation for the substrate at the next polishing station. For example, if the characterizing value indicates that the etch trench depth is greater, the post thickness target for the subsequent polishing station can be adjusted with more removal amount to keep the remaining metal line thickness constant. If the characterizing value indicates that the underlying layer thickness has changed, the reference spectrum for in-situ endpoint detection at the subsequent polishing station can be modified so that endpoint occurs closer to the target metal line thickness.

In some implementations, the controller 190 uses the characterizing value to adjust the polishing operation for a subsequent substrate at the previous polishing station. For example, if the characterizing value indicates that the etch trench depth is greater, the post thickness target for the previous polishing station can be adjusted with more removal amount to keep the remaining metal line thickness constant. If the characterizing value indicates that the underlying layer thickness has changed, the reference spectrum for in-situ endpoint detection at the previous polishing station can be modified so that endpoint occurs closer to the target metal line thickness.

In some implementations, the controller 190 analyzes the measured spectra and determines the proper substrate route. For example, the controller 190 can compare the characterizing value to a threshold, or determine whether the characterizing value falls within a predetermined range. If the characterizing value indicates that polishing is incomplete, e.g., if it falls within the predetermined range indicating an underpolished substrate or does not exceed a threshold indicating a satisfactorily polished substrate, then the substrate can be routed back to previous polishing station for rework. Once the rework is completed, the substrate can be measured again at the metrology station, or transported to the next polishing station. If the characterizing value does not indicate that polishing is incomplete, the substrate can be transported to the next polishing station.

For example, a parameter such as metal residue can be measured using wet metrology at the in-sequence metrology station 180. If metal residue detected, the substrate can be routed back to previous polishing station for rework. Otherwise, the substrate can be transported to the next polishing station.

In order to detect metal residue, the controller 190 can evaluate the percentage of the area that is covered by the filler material, each measured spectrum 300 is compared to a reference spectrum. The reference spectrum can be the spectrum from a thick layer of the filler material, e.g., a spectrum from a metal, e.g., a copper or tungsten reference spectrum. The comparison generates a similarity value for each measured spectrum 300. A single scalar value representing the amount of filler material within the area 18 can be calculated from the similarity values, e.g., by averaging the similarity values. The scalar value can then be compared to a threshold to determine the presence and/or amount of residue in the area.

In some implementations, the similarity value is calculated from a sum of squared differences between the measured spectrum and the reference spectrum. In some implementations, the similarity value is calculated from a cross-correlation between the measured spectrum and the reference spectrum.

For example, in some implementation a sum of squared differences (SSD) between each measured spectrum and the reference spectrum is calculated to generate an SSD value for each measurement spot. The SSD values can then be normalized by dividing all SSD values by the highest SSD value obtained in the scan to generate normalized SSD values (so that the highest SSD value is equal to 1). The normalized SSD values are then subtracted from 1 to generate the similarity value. The spectrum that had the highest SSD value, and thus the smallest copper contribution, is now equal to 0.

Then the average of all similarity values generated in the prior step is calculated to generate the scalar value. This scalar value will be higher if residue is present.

As another example, in some implementation a sum of squared differences (SSD) between each measured spectrum and the reference spectrum is calculated to generate an SSD value for each measurement spot. The SSD values can then be normalized by dividing all SSD values by the highest SSD value obtained in the scan to generate normalized SSD values (so that the highest SSD value is equal to 1). The normalized SSD values are then subtracted from 1 to generate inverted normalized SSD values. For a given spectrum, if the inverted normalized SSD value generated in the previous step is less than a user-defined threshold, then it is set to 0. The user-defined threshold can be 0.5 to 0.8, e.g., 0.7. Then the average of all values generated in the prior step is calculated to generate the scalar value. Again, this similarity value will be higher if residue is present.

If the calculated scalar value is greater than a threshold value, then the controller 190 can designate the substrate as having residue. On the other hand, if the scalar value is equal or less than the threshold value, then the controller 190 can designate the substrate as not having residue.

If the controller 190 does not designate the substrate as having residue, then the controller can cause the substrate to be processed at the next polishing station normally. On the other hand, controller 190 designates the substrate as having residue, then the controller can take a variety of actions. In some implementations, the substrate can be returned immediately to the previous polishing station for rework. In some implementations, the substrate is returned to the cassette (without being processed at a subsequent polishing station) and designated for rework once other substrates in the queue have completed polishing. In some implementations, the substrate is returned to the cassette (without being processed at a subsequent polishing station), and an entry for the substrate in a tracking database is generated to indicate that the substrate has residue. In some implementations, the scalar value can be used to adjust a subsequent polishing operation to ensure complete removal of the residue. In some implementations, the scalar value can be used to flag the operator that something has gone wrong in the polishing process, and that the operator's attention is required. The tool can enter into a number of error/alarm states, e.g. return all substrates to a cassette and await operator intervention.

In another implementation, the calculated similarity value for each measurement value is compared to a threshold value. Based on the comparison, each measurement spot is designated as either filler material or not filler material. For example, if an inverted normalized SSD value is generated for each measurement spot as discussed above, then the user-defined threshold can be 0.5 to 0.8, e.g., 0.7.

The percentage of measurement spots within the area 18 that are designated as filler material can be calculated. For example, the number of measurement spots designated as filler material can be divided by the total number of measurement spots.

This calculated percentage can be compared to a threshold percentage. The threshold percentage can be calculated either from knowledge of pattern of the die on the substrate, or empirically by measuring (using the measurement process described above) for a sample substrate that is known to not have residue. The sample substrate could be verified as not having residue by a dedicated metrology station.

If the calculated percentage is greater than the threshold percentage, then the substrate can be designated as having residue. On the other hand, if the percentage is equal or less than the threshold percentage, then the substrate can be designated as not having residue. The controller 190 can then take action as discussed above.

In some implementations a probe 180′ of an optical metrology system 160 is positioned between the loading and unloading station and one of the polishing stations. If the probe 180′ is positioned between the loading station and the first polishing station, then a characterizing value can be measured by the metrology system and fed forward to adjust polishing of the substrate at first polishing station. If the probe 180′ is positioned between the last polishing station and the unloading station, then a characterizing value can be measured by the metrology system and fed back to adjust polishing of a subsequent substrate at the last polishing station, or if residue is detected then the substrate can be sent back to the last polishing station for rework.

The control schemes described above can more reliably maintain product substrates within manufacture specification, and can reduce rework, and can provide rerouting of the substrate to provide rework with less disruption of throughput. This can provide an improvement in both productivity and yield performance.

The above described polishing apparatus and methods can be applied in a variety of polishing systems. For example, rather than be suspended from a track, multiple carrier heads can be suspended from a carousel, and lateral motion of the carrier heads can be provided by a carriage that is suspend from and can move relative to the carousel. The platen may orbit rather than rotate. The polishing pad can be a circular (or some other shape) pad secured to the platen. Some aspects of the endpoint detection system may be applicable to linear polishing systems (e.g., where the polishing pad is a continuous or a reel-to-reel belt that moves linearly). The polishing layer can be a standard (for example, polyurethane with or without fillers) polishing material, a soft material, or a fixed-abrasive material. Terms of relative positioning are used; it should be understood that the polishing surface and substrate can be held in a vertical orientation or some other orientations.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. 

What is claimed is:
 1. A method of controlling a polishing system, comprising: polishing a substrate at a first polishing station; transporting the substrate to an in-line optical metrology system positioned between the first polishing station and a second polishing station; at the in-line optical metrology system measuring a spectrum reflected from the substrate, and generating a characterizing value from the spectrum; determining that the substrate needs rework based on the characterizing value; returning the substrate to the first polishing station and performing rework of the substrate at the first polishing station; and transporting the substrate to the second polishing station and polishing the substrate at the second polishing station.
 2. The method of claim 1, wherein the substrate is held by a carrier head and the carrier head is suspended from a track, and the transporting the substrate is performed by moving the carrier head along the track.
 3. The method of claim 1, wherein generating the characterizing value comprises obtaining a plurality of measured spectra with the optical metrology system from a plurality of different measurement spots within an area on the substrate, generating a plurality of values based on the plurality of measured spectra, and combining the values to generate the characterizing value.
 4. The method of claim 3, wherein generating a plurality values comprises comparing each of the plurality of measured spectra to a reference spectrum to generate a similarity value.
 5. The method of claim 3, wherein the substrate comprises a plurality of dies, and the area is substantially equal to an area of one of the dies.
 6. The method of claim 1, wherein generating the characterizing value comprises at least one of identifying a matching reference spectrum from a library of reference spectra and determining the characterizing value associated with the matching reference spectrum, determining a wavelength or width of a peak or valley in the spectrum, or fitting an optical model to the spectrum.
 7. The method of claim 1, comprising measuring another spectrum reflected from the substrate at the in-line optical metrology system positioned between the first polishing station and a second polishing station after performing rework and before transporting the substrate to the second polishing station.
 8. The method of claim 1, wherein the polishing the substrate at the first polishing station comprises a filler layer clearing recipe, and polishing the substrate at the second polishing station comprises an underlying layer polishing recipe.
 9. The method of claim 1, wherein polishing the substrate at the first polishing station comprises a bulk polishing step of a copper damascene process.
 10. The method of claim 9, wherein polishing the substrate at the second polishing station comprises a dielectric exposure step of a copper damascene process.
 11. The method of claim 1, comprising transporting the substrate to a cassette and performing polishing of at least one other substrate at the first polishing station before returning the substrate to the first polishing station and performing rework of the substrate at the first polishing station.
 12. A polishing system, comprising: a first polishing station including a first support for a first polishing pad; a second polishing station including a second support for a second polishing pad; a carrier head to hold a substrate, the carrier head supported by a support structure and movable between the first polishing station and the second polishing station; an in-line optical metrology system positioned between the first polishing station and the second polishing station, the optical metrology system configured to measure a spectrum reflected from the substrate and generate a characterizing value from the spectrum; and a controller configured to cause the carrier head to move to the first polishing station, to cause the substrate to be polished at the first polishing station, to cause the carrier head to move to the in-line metrology system, to determining whether the substrate needs rework based on the characterizing value, to cause the carrier head to return to the first polishing station and performing rework of the substrate at the first polishing station, to cause the carrier head to move to the second polishing station, and to cause the substrate to be polished at the second polishing station.
 13. The system of claim 12, wherein the support structure comprises a track and the carrier head is movable along the track.
 14. The system of claim 13, wherein the carrier head is suspended from a carriage on the track.
 15. The system of claim 12, wherein the controller is configured to cause the carrier head to move to the in-line optical metrology system and to receive another spectrum reflected from the substrate at the in-line optical metrology system after performing rework and before transporting the substrate to the second polishing station.
 16. The system of claim 12, comprising a cassette and a robot to transfer the substrate from the carrier head to the cassette, and wherein the controller is configured to cause the robot to transport the substrate from the carrier head to the cassette before the substrate is returned to the first polishing station and rework of the substrate is performed at the first polishing station.
 17. The system of claim 16, wherein the controller is configured to cause polishing of at least one other substrate at the first polishing station before the substrate is returned to the first polishing station and rework of the substrate is performed at the first polishing station. 